Over 69 percent of the U.S. population is currently overweight or obese; 35 percent are obese. Obesity is a primary cause of numerous chronic diseases including type II diabetes, cancer, heart disease and the metabolic syndrome. Behavioral weight loss interventions have successfully resulted in significant clinical improvements in weight (7-10 percent weight loss) that is sustained for at least 6-12 months. Unfortunately, such interventions are often time consuming, resource intensive and difficult to scale. Digital health holds enormous promise for addressing weight loss and several trials have shown meaningful weight loss using digital health tools. Digital health provides a means of reaching large numbers of people with highly personalized content and sustained long-term participant engagement -all at low cost. Despite the promise, digital health has not delivered. Evidence-based tools are virtually absent from the commercial digital health space. This application creates a new commercial market segment for digital health - health application programming interfaces. An API is a way for software applications to talk to one another in a language that they both understand. APIs are efficient, cost effective, and ubiquitous in the commercial market. The API allows us to disseminate evidence based interventions in an efficient, cost-effective manner. APIs allow us to disseminate effective interventions in developer's language, while preserving the interventions' theoretical and evidentiary basis. APIs are used anywhere that software is used, so they will allow us to reach varied health market segments including app developers, disease management firms, health insurers and employee wellness firms. In Phase I, we will create a commercial weight loss API (WeightBox) that builds on an existing evidence based framework and evaluate its usability. In Phase II, we will test the effectiveness of a mobile app that uses the WeightBox API in a randomized clinical trial. At the end of Phase II, our API (and proof-of-principle app) might be the only digital healt weight loss tools in the market with clinical trial effectiveness evidence.